Carpal tunnel syndrome is a common problem that continues to increase in incidence. Symptoms of the syndrome include motor weakness and sensory loss in areas of the hand innervated by the median nerve. Such symptoms include paresthesias, burning or tingling in the radial-palmar aspect of the hand (the thumb and adjacent fingers), and pain in the wrist, palm or even the forearm. Sensory deficits such as numbness may occur in the palmar aspect of the first three digits and motor symptoms such as weakness or inability to oppose the thumb and fingers may follow. The syndrome may occur in one or both hands, and occurs more frequently in women than in men. The syndrome often occurs in people who use their hands for repetitive motions, such as typing, computer data entry, housepainting, carpentry, and associated occupations.
Carpal tunnel syndrome is believed to be caused by compression of the median nerve that supplies sensation to the thumb, index finger, middle finger and radial half of the ring finger. The nerve passes through the wrist beneath the flexor retinaculum or transverse carpal ligament that overlies and protects the wrist just below the palm. The transverse carpal ligament and tendons of the flexor muscles define a tunnel through which the median nerve passes. If the nerve becomes swollen as it passes through the carpal tunnel, or if the carpal ligament becomes abnormally thickened, the nerve may be compressed and function abnormally.
Previous studies have been performed to assess the effects of elevated pressure on the median nerve in the carpal tunnel. Gelberman et al. used the wick-catheter technique to measure changes in pressure with the wrist in various positions. Patients who had carpal tunnel syndrome had mean pressures of ninety-nine millimeters of mercury (13.2 kilopascals) with the wrist flexed and 110 millimeters of mercury (14.7 kilopascals) with the wrist extended. For control subjects, the measurements were thirty-one millimeters of mercury (4.1 kilopascals) with the wrist flexed and thirty millimeters of mercury (4 kilopascals) with the wrist extended.
Lundborg et al., J. Hand Surg. 7:252-259 (1982) studied the effects of prolonged compression of the median nerve in sixteen volunteers who had no history of carpal tunnel syndrome. Compression with pressure of thirty to ninety millimeters of mercury (4 to 12 kilopascals) for thirty to ninety minutes produced abnormal delays in nerve conduction and abnormal two-point discrimination. These investigators and others have theorized that compression produces ischemia of the median nerve, resulting in paresthesias and reversible failure of nerve conduction. The earlier indicator of impaired function of a nerve due to compression was a delay in sensory conduction.
Gellman et al., J. Bone and Joint Surg. 68A:735-737 (1986) performed sensibility testing with the Semmes-Weinstein monofilament test in patients who had carpal tunnel syndrome. They found the sensitivity of the test to be 91%,but they also found a 21% rate of false-positive results in a normal control population.
An accurate diagnosis of carpal tunnel syndrome must be made if appropriate treatment is to be obtained. It is critical, for example, to distinguish carpal tunnel syndrome from a C6 root compression due to cervical osteoarthropathy. Symptoms resembling carpal tunnel syndrome can also accompany neuromuscular disease, cerebral damage, peripheral neuropathy, or may be psychogenic in origin. Up to the present time, diagnosis of the syndrome has been made using Phelan's Test, Tinel's Sign, and electromyographic nerve conduction studies.
Phelan's Test requires that the patient's wrist be held in acute flexion for sixty seconds, or that the patient presses the back of both hands together to form right angles. If numbness and tingling develop over the distribution of the median nerve, the sign is positive and carpal tunnel syndrome is suspected. To elicit Tinel's Sign, an examiner lightly percusses the course of the median nerve in the carpal tunnel for several seconds without exerting sustained pressure. A tingling or electric sensation in the distribution of the median nerve is a positive test that additionally suggests carpal tunnel syndrome.
Phelan's maneuver and Tinel's Sign, however, are of limited diagnostic value. Although they suggest the presence of the syndrome, the diagnosis must be confirmed by expensive electromyographic (EMG) nerve conduction studies that measure the velocity of nerve conduction through the carpal tunnel. Reduced sensory and motor conduction velocity is associated with carpal tunnel syndrome. An EMG diagnosis is sensitive enough to detect the syndrome in 85% of patients, but a normal EMG study can still occur in patients who have anatomic evidence of a median nerve compression.
The value of a screening test is judged by its sensitivity and specificity. The sensitivity of a test measures the likelihood of a positive result in patients known to have the disease. Conversely, specificity is the likelihood of a negative result in patients known to be free of the disease. Previous studies have shown that the sensitivity and specificity of the Phelan's and Tinel's tests are relatively low, such that expensive EMG studies are required to make an accurate diagnosis. Even the more sophisticated EMG studies, however, have a sensitivity of only about 85%. Hence, there is a need in clinical medicine for a quick screening test that will accurately diagnose carpal tunnel syndrome.
It is accordingly an object of this invention to provide an improved, inexpensive, sensitive test for screening patients who are complaining of the symptoms of carpal tunnel syndrome.
It is another object of the invention to provide such an improved test that will be more specific for diagnosing carpal tunnel syndrome in those patients who have anatomic nerve compressions, without also being positive for a large number of patients who do not have anatomic evidence of the syndrome.
Yet another object of the invention is to help reduce medical costs by providing an improved clinical screening test that can quickly be performed in an office setting without resort to expensive electromyographic studies.
It is also an object of the invention to provide a device that is suitable for performing the improved test.
These and other objects of the invention will be better understood by reference to the following drawings and detailed description.